Enhanced luminescence from spontaneously ordered Gd2O3:Eu3+ based nanostructures

Applied Surface Science 255 (2009) 9112–9123

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Enhanced luminescence from spontaneously ordered Gd2O3:Eu3+ based nanostructures Geo Rajan, K.G. Gopchandran * Department of Optoelectronics, University of Kerala, Kariavattom, Thiruvananthapuram 695581, India

A R T I C L E I N F O

A B S T R A C T

Article history: Received 3 January 2009 Received in revised form 18 June 2009 Accepted 26 June 2009 Available online 4 July 2009

Nanostructured Gd2O3:Eu3+ and Li+ doped Gd2O3:Eu3+ thin films were prepared by pulsed laser ablation technique. The effects of annealing and Li+ doping on the structural, morphological, optical and luminescent properties are discussed. X-ray diffraction and Micro-Raman investigations indicate a phase transformation from amorphous to nanocrystalline phase and an early crystallization was observed in Li+ doped Gd2O3:Eu3+ thin films on annealing. AFM images of Li+ doped Gd2O3:Eu3+ films annealed at different temperatures especially at 973 K show a spontaneous ordering of the nanocrystals distributed uniformly all over the surface, with a hillocks (or tips) like self-assembly of nanoparticles driven by thermodynamic and kinetic considerations. Enhanced photoemission from locations corresponding to the tips suggest their use in high resolution display devices. An investigation on the photoluminescence of Gd2xEuxO3 (x ¼ 0:10) and Gd2xyEuxLiyO3 (x ¼ 0:10, y ¼ 0:08) thin films annealed at 973 K reveals that the enhancement in luminescence intensity of about 3.04 times on Li+ doping is solely due to the increase in oxygen vacancies and the flux effect of Li+ ions. The observed decrease in the values of asymmetric ratio from the luminescence spectra of Li+ doped Gd2O3:Eu3+ films at high temperature region is discussed in terms of increased Eu–O bond length as a result of Li+ doping. ß 2009 Elsevier B.V. All rights reserved.

Keywords: Thin films Optical properties Surface properties Photoluminescence spectroscopy

1. Introduction Luminescent light emitting materials based on lanthanide phosphors are emerging rapidly and are revolutionizing the display and lighting industries. It is anticipated that oxide phosphors have the potential for replacing conventional displays. However, significantly improved performance of displays demands high quality phosphors having sufficient brightness and long term stability. Many authors have used rare-earth doped oxide particles as phosphors in various display applications such as field emission displays (FEDs) [1], plasma display panels (PDPs) [2], cathode ray tubes (CRT) [3], and more. Lanthanide activated rare-earth oxides [4,5] remain as promising materials for next generation display technology because of several essential superior properties such as luminescent characteristics, stability in vacuum and corrosion free gas emission under electron bombardment compared with traditional cathode ray tube red phosphors used in current field emission displays [6,7]. The field emission display have gained a great interest and have been recognized as one of the most promising technologies in flat panel display market due to their

* Corresponding author. E-mail address: [email protected] (K.G. Gopchandran). 0169-4332/$ – see front matter ß 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2009.06.128

most important features like great brightness, wide horizontal and vertical view angles, good contrast ratio, high efficiency with low power consumption, short response time and wide work temperature range [8–12]. Additionally FEDs have high brightness (3000 cd m2) and very high efficiency (2 W/10.4 in.). The only problem that appears in FEDs is that the operating time is shorter than in the case of PDPs and LCDs. This problem is connected with progressive degradation of vacuum level in the display space [13– 15]. The three main sources of gases in FEDs are the gas emitted during device encapsulation process [14], the employment of inappropriate materials in the device construction [13,16] and the third and most important source of gases is improper type of phosphors [13,15,16]. The above considerations lead to a conclusion that new class of phosphors need to be applied in field emission displays. The rare-earth doped nanocrystalline oxides seem to be very promising candidates for FED phosphor applications. Among the oxide based phosphors, Gd2O3:Eu3+ thin films were proposed as one of the most promising oxide based red phosphor systems. Due to the 5 D0 –7 F2 transition within europium, Gd2O3:Eu3+ shows red luminescence properties and emits red light at 612 nm wavelength [17,18]. In thin film phosphors brightness may be associated with several factors such as (i) the interaction between the generated beam material and substrate,

Geo Rajan, K.G. Gopchandran / Applied Surface Science 255 (2009) 9112–9123

(ii) the film processing conditions, and (iii) the composition of the film materials. Among these factors composition could be one of the lead breakthroughs for increased brightness of Gd2O3:Eu3+ thin films. It is well known that even in very small quantities, the Li+ coactivators frequently play an important role in the enhancement of the luminescent efficiency of phosphors [4,19]. Yi et al. reported that the photoluminescent (PL) brightness of Li+ doped Gd2O3:Eu3+ films are 2.1–2.3 times greater than that of undoped Gd2O3:Eu3+ films [19–21]. They suggested that the improved PL brightness that resulted from Li+ doping is not only due to changes in the crystalline phase but also the reduced internal reflection that occurs for rougher surfaces. Park et al. reported that co-doping with Li+ ions leads to an increase in the quantum yield (83–92%) of Gd2O3:Eu3+ powders [22]. They suggested that Li+ substitution in the lattice leads to a decrease in interstitial oxygen and hence an increase in the hole concentration and suggested that the decrease in interstitial oxygen also leads to a decrease in competitive absorption and hence to a higher quantum yield. In previous studies nano-seized Gd2O3:Eu3+ phosphor materials have been prepared using solid state reactions [23], hydro thermal method [24], co-precipitation methods [25], sol–gel methods [26,27], spray pyrolysis [28], chemical vapour deposition [29] and pulsed laser deposition technique(PLD) [18,20,21]. In PLD, one can control size distribution and shape of nanocrystals by varying the parameters like target to substrate distance, laser fluence, back ground gas pressure, substrate temperature, etc.; and thus it emerges as an effective tool for the growth of quantum structures with high chemical purity and controlled stoichiometry [30]. In this work, we report on a comprehensive study of the effect of annealing on the Li+ doped and undoped Gd2O3:Eu3+ films prepared by PLD, consequent structural, morphological and optical properties and how it affects the chromaticity of the phosphor. In the present investigation, the photoluminescent intensity of Li+ doped Gd2O3:Eu3+ thin films is obtained as about 3.04 times higher than that of undoped Gd2O3:Eu3+ films and the chromaticity coordinates shifted from (x ¼ 0:562, y ¼ 0:437) to (x ¼ 0:577, y ¼ 0:422) and this will overcome the drawback of Gd2O3:Eu3+ that its emission peak is around 612 nm (orange-red region), which does not completely satisfy the requirement of saturated colours in full colour displays. 2. Experimental Gd2xEuxO3 (x ¼ 0:10) and Gd2xyEuxLiyO3 (x ¼ 0:10, y ¼ 0:08) powder samples (were of 99.99% purity, Sigma–Aldrich) were prepared from stoichiometric amounts of Gd2O3, Eu2O3 and Li2O3. For a ceramic target, the powder mixture was pelletized into a disc and sintered at 1623 K for 10 h. The films were prepared on amorphous fused quartz substrates by the pulsed laser deposition (PLD) technique, using a Q-switched Nd:YAG laser (Quanta-ray INDI-Series, Spectra-Physics) with 12 J cm2 laser fluence at 532 nm, pulse width 8 ns, and repetition frequency 10 Hz. Target substrate—distance was 6 cm and the deposition time was 30 min. The target was rotated with constant speed to avoid pitting of the target at any given spot and to obtain uniform ablation. The films were deposited at room temperature under a vacuum of 106 mbar and subsequently annealed at different temperatures (AT ) up to 1273 K, for a period of 1 h. The heat treatment consisted of raising the temperature at a rate of 5 K min1, then maintaining the temperature for 1 h and gradually lowering it to room temperature. The crystallinity of thin film phosphors were examined using X-ray diffraction (XRD) (XPERT PRO Diffractometer) measurements employing Cu K a radiation with a wavelength of 0.15406 nm. Micro-Raman spectra of the films were recorded using labram-HR 800 spectrometer equipped with an excitation source of laser radiation at a wavelength of 488 nm from an argon

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ion laser. Spectra were acquired by 1800 greeds/mm grating, a super-notch filter having a cut-off at 50 cm1 and a Peltier cooled CCD camera, allowing a spectral resolution of about 1 cm1. The surface morphology of the films were investigated by scanning electron microscopy (SEM) (Sirion). Surface morphology of the deposited films at nanometric scale were investigated by AFM (Digital Instruments Nanoscope E) measurements in contact mode. Particle size and root mean square (rms) surface roughness of the deposited films were determined on a scan area of 500 nm  500 nm. Optical measurements were performed in the wavelength range from 300 to 900 nm using a double beam UV–vis spectrophotometer, Jasco-D550. Photoluminescence spectra of the samples were recorded by Horiba Jobin Yvon Flourolog (III) modular spectroflourometer equipped with 450 W Xenon lamp and Hamatsu R928-28 photomultiplier. 3. Results and discussion 3.1. XRD analysis Fig. 1(a) shows the XRD patterns of Li+ doped Gd2O3:Eu3+ films deposited on quartz substrates and after subsequent annealing at different temperatures ranging from 300 to 1073 K. Peaks from the cubic and monoclinic structures are labeled as C and M, respectively with miller indices in the parenthesis. The XRD pattern of the as deposited film was found to be amorphous. All the peaks in the diffraction pattern were indexed according to the ASTM data card No.: 88-2165 for cubic and No.: 43-1015 for monoclinic Gd2O3. The films annealed at temperatures 773, 873 and 973 K show a noticeable preference for the h1 1 1i cubic crystallographic orientation. The intensity of the XRD peak from (2 2 2) crystal plane was found to increase with annealing temperatures in the range from 773 to 973 K and this can be attributed to the enhanced oxidation kinetics and improvement in crystalline nature of the films. No Eu2O3 and Li2O3 diffraction peaks were detected, which means the Eu3+ and Li+ ions incorporated into the Gd2O3 host lattice homogeneously. Peaks from a mixture of cubic and monoclinic phases were detected from films annealed at a temperature of about 1073 K, with a preferential orientation along (2 0 2) lattice plane reflection corresponding to monoclinic crystalline phase. The crystalline structures of Gd2O3 and Eu2O3 were examined by Roth and Schneider [31] and they found that Gd2O3 and Eu2O3 were crystallized in cubic C-type structure at low temperature and inverted directly and irreversibly to monoclinic B-type structure at about 1498 and 1348 K respectively [31,32]. Generally in nanocrystalline thin film structures, the predominant structure or orientation mainly depends on the processing parameters. The structural changes occur by surface diffusion and migration of grain boundaries during the coalescence of two differently oriented nuclei. In such cases, smaller nuclei may easily rotate on coalescence that induces the structural changes [33]. The various factors that influence the stable nanocrystalline state of a material include the lowest surface energy, grain boundary energy and the diffusion of surface atoms [34]. The reports by Lee et al. reveal that the strain energy minimization and surface energy minimization would compete with one another to determine the preferred orientation of grain growth and final texture of thin films [35]. Fig. 1(b) shows the XRD patterns of the Gd2O3:Eu3+ films annealed at 973 and 1173 K and Li+ doped Gd2O3:Eu3+ films annealed at 973 K. All the films exhibit cubic structure with the preferential orientation along (2 2 2) lattice plane. It was found that the full width at half maximum (FWHM) decreases almost 50% for films annealed at 973 K, when lithium doping was done. This is an indication of the better crystallization and larger grain size and can be regarded as a result of the flux effect of Li+ ions during the growth process, which plays a role in effective promotion of the

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Fig. 1. XRD patterns of films annealed at different temperatures: (a) Li+ doped Gd2O3:Eu3+, (b) Li+ doped and undoped Gd2O3:Eu3+ films.

incorporation of Eu2O3 and Gd2O3 as well as Li+ ions themselves into the host lattice [36]. The grain size (dc ) of the films were estimated using Debye– Sherrer relation [37] and is given in Table 1. The grain size of the Li+ doped Gd2O3:Eu3+ film was found to increase from 5.41 nm at an annealing temperature of about 773 K to 41.3 nm at 973 K. On annealing at 1073 K the grain size decreases to about 25.4 nm with a change in crystalline phase from cubic at 973 K to monoclinic phase at 1073 K. The effective ionic radius of Li+ (0.76 A˚) ion is smaller than that of Gd3+ ion (0.94 A˚). So the Li+ ions are suitable for occupation of the Gd3+ sites, which will give rise to a number of oxygen vacancies for the charge neutrality [38]. But during annealing at elevated temperatures, few oxygen vacancies are filled and at the same time more and more oxygen vacancies are created due to the loss of volatile lithium components. The observed reduction in grain size can be attributed to the defects related with oxygen vacancies, which are formed with the increase of voids due to the loss of volatile lithium components at elevated annealing temperatures. Zhi et al. reported similar reduction in grain size after subsequent annealing at elevated temperatures due to the defects related with oxygen vacancies in ZnO thin films prepared by plasma enhanced chemical vapour deposition [39]. In the case of Gd2O3:Eu3+ films the grain size was found to be 20.83 nm at 973 K and 43.6 nm at 1173 K. The grain size was increased more than twice over at 973 K due to the Li+ incorporation into the Gd2O3:Eu3+ matrix. Similar increase in Table 1 Summary of X-ray diffraction analysis and extinction coefficient of Li+ doped and undoped Gd2O3:Eu3+ thin films. AT (K)

FWHM ( )

Strain (T) 103

Li doped Gd2O3:Eu3+ films 300 – – 773 1.5141 11.8240 873 0.2414 1.8708 973 0.1985 1.5070 1073 0.3204 2.7110 Gd2O3:Eu3+ films 973 0.3940 1173 0.1879

2.9570 1.4340

(e) 103

dc (nm)

dp (nm)

– 5.36 7.21 7.21 –

– 5.41 33.9 41.3 25.4

– 35 45 60 30

0.1439 0.1363 0.1267 0.1770 0.1907

13.80 8.86

20.83 43.6

213 176

0.1006 0.1275

Extinction coefficient (k) at 550 nm

grain size with Li+ doping in Gd2O3:Eu3+ matrix has been reported by Yi et al. [20]. On comparison with Gd2O3:Eu3+ films annealed at 1173 K and Li+ doped Gd2O3:Eu3+ films annealed at 973 K, Li+ ion incorporation reduces the crystallization temperature of Gd2O3:Eu3+ films from 1173 to 973 K. Lanthanide sesquioxides are known to crystallize in various structures according to the radius of the rare-earth ion. Gadolinium oxide crystallizes in a cubic C-type structure below 1473 K and is transformed at high temperature in to a monoclinic B-type structure [31,32]. The appearance of monoclinic phase at a lower annealing temperature can be attributed to the early crystallization resulted from the flux effect of Li+ ions [36]. Using the biaxial strain model, the strain

e¼

C film  C bulk C bulk

(1)

where C film is the lattice constant of the film and C bulk is the unstrained bulk lattice parameter in the direction of the C axis and were evaluated for all the films and are given in Table 1. The lattice strain (T) of all the films was calculated using the relationship [40] Ttan u ¼

l D cos u

 bh k l

(2)

where l is the wavelength of the X-ray radiation and bh k l is the FWHM of the peak considered and 2u is its angular position. It is seen from Table 1, that the lattice strain in Li+ doped Gd2O3:Eu3+ thin films decreases with increase in annealing temperature and is minimum at 973 K and then the strain increases. Li+ doped Gd2O3:Eu3+ thin film sample annealed at 973 K have the less lattice distortion and the matrix is at a more stable structure; the cubic structure as far as the other Li+ doped samples at different annealing temperatures are concerned. Gd2O3:Eu3+ thin film sample annealed at 1173 K shows a lower lattice strain as compared with Li+ doped Gd2O3:Eu3+ thin film sample annealed at 973 K. This indicates that the incorporation of Li+ ions into the Gd2O3 matrix will lead to the lattice distortion, which causes a strong tendency of the matrix to form a more stable structure, i.e., the cubic phase [41]. It can be observed from Table 1 that the biaxial strain (e) is compressional in nature. This may be owing to the two dimensional grain growths due to higher interaction between the film and substrate at lower film thickness. Tamule-

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Fig. 2. Micro-Raman spectra of Li+ doped Gd2O3:Eu3+ films annealed at different temperatures.

vicius et al. reported that the compressional strain could dominate in the two dimensional growth mode [42]. 3.2. Micro-Raman studies Micro-Raman measurements were performed for the samples since this technique is a powerful tool for phase and structural analysis of materials. Fig. 2 shows the Micro-Raman spectra of Li+ doped Gd2O3:Eu3+ films annealed at different temperatures. The Raman peaks were observed at 357 cm1 for Li+ doped Gd2O3:Eu3+ thin films annealed at 773 and 873 K. The intense Raman peaks for the samples annealed at 973 and 1073 K were at 358 and 463 cm1 respectively. Fig. 3 shows the Micro-Raman spectra of undoped Gd2O3:Eu3+ thin film samples annealed at 973 and 1173 K and Li+ doped sample annealed at 973 K. In this figure, the strongest Raman peaks were observed at 353 and 357 cm1 for undoped Gd2O3:Eu3+ thin film samples annealed at temperatures 973 and 1173 K respectively. The major peak around 360 cm1 has been assigned as the Fg +Eg mode and these Raman bands mainly attributed to the cubic C-type structure with space group Ia3 of Gd2O3 matrix, was also confirmed by the XRD analysis in Section 3.1[43–46]. For the film annealed at 1073 K, the strong and intense Raman peak at 463 cm1, has been assigned as the Ag mode and is ascribed as the monoclinic B-type structure of Gd2O3[47]. The monoclinic nanocrystalline orientation can modify the respective intensity of the bands in the film and may be attributed to the increased intensity of the 463 cm1 peak [43]. All the annealed films except the Li+ doped Gd2O3:Eu3+ film annealed at 1073 K, show a down shift by few wave numbers as a consequence of the nanosize of the samples in the present study [48].

Zhang et al. have pointed out that the decrease in crystalline dimension to the nanometer scale can cause the frequency shift and broadening of Raman peaks as a result of phonon confinement. In nanocrystals the phonons are spatially confined and the phonons all over the Brillouin Zone will contribute to the first order Raman scattering due to the breakdown of the phonon momentum selection rule q  0 [49]. The FWHM of the Raman peaks around 360 cm1 decrease from 17.63 to 6.40 cm1 with

Fig. 3. Micro-Raman spectra of undoped Gd2O3:Eu3+ thin film samples annealed at 973 and 1173 K and Li+ doped sample annealed at 973 K.

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Fig. 4. Scanning electron microscopic images of Gd2O3:Eu3+ thin film samples: (a) as deposited; annealed at (b) 973 K, (c and d) 1073 K, (e and f) 1173 K and (g and h) 1273 K.

increase in grain size from 5.41 to 41.3 nm in the annealing temperature range from 773 to 973 K in Li+ doped Gd2O3:Eu3+ thin films. Similar effects were observed in undoped Gd2O3:Eu3+ thin films annealed at 973 and 1173 K. This asymmetric broadening and shift of Raman lines with reduction in grain size within the nano regime is a consequence of phonon confinement and dispersion. FWHM of the peak around 360 cm1 of Gd2O3:Eu3+ thin film annealed at 973 K decreased considerably in Li+ doped Gd2O3:Eu3+ thin film annealed at the same temperature and this may be due to the improved crystallinity of the sample with Li+ doping. It was reported that Raman vibration bands shift to lower wave numbers with increase of strain. Similar effects have been reported by Liao et al. [50] and Portinha et al. [51] in IrO2 and ZrO2 films. Here we assume that the downward shift observed in the characteristic Raman band of Gd2O3 is a result of competitive mechanism between higher frequency shifts due to grain size reduction and lower frequency shift to strain in the films, with the predominance of grain size reduction over the frequency shifts. 3.3. SEM and AFM analysis Scanning electron microscopic images of Gd2O3:Eu3+ thin film samples annealed at different temperatures are shown in Fig. 4. The change in surface morphology as a function of annealing temperature is mainly due to the realization of different surface energy and surface migration properties leading to the formation of surface structures and crystallites of varying size and nature. With increase in annealing temperature the films were found to transform into crystalline form with different degree of ordering. The morphology of the film annealed at 1173 K consists of clusters of nanocrystals agglomerated together with voids in between, and this film was found to exhibit intense photoemission among the undoped films. However further annealing at 1273 K (Fig. 4g and h) transformed the film into one with morphology consisting of closely packed elongated nanostructures. Atomic force microscopic images of Li+ doped Gd2O3:Eu3+ thin film samples annealed at different temperatures are shown in Fig. 5. The AFM image of the film annealed at 773 K shows a transformation stage from amorphous to nanocrystalline phase. It can be seen that annealing leads to the formation of spontaneously ordered nanostructures, driven by thermodynamic and kinetic contemplation. The appearance of this order depends on the one hand, in general, on the symmetry and dimensionality of the

system and on the other hand crucially on the individual details of the underlying microscopic interactions between the grains. AFM images of the films annealed at different temperatures show a spontaneous ordering of the nanocrystals distributed uniformly all over the surface, with a hillocks (or tips) like self-assembly of nanograins. The tips that were formed seen only when the film changes from amorphous to nanocrystalline form. The AFM image of the film annealed at 973 K consisted of well defined hillocks uniformly distributed all over the film surface. The formation of hillocks in Li+ doped Gd2O3:Eu3+ thin films can be attributed to the relaxation of thermal expansion mismatch stresses between the substrate and the film. The compressive stress that develops during heating results in diffusion of atoms, either through the lattice or along grain boundaries, leading to the formation of hillocks [52]. The variation in the self assembly of nanocrystals exhibited by the films especially at annealing temperatures 873 and 1073 K is an indication of the on-going phase change as evidenced from XRD analysis. We report the arrays of Li+ doped Gd2O3:Eu3+ hillocks with enhanced photoemission from locations corresponding to the tips suggest that it enable to use in field emission display applications with high resolution and tunable chromaticity. 3.4. Optical properties Fig. 6 shows the wavelength dependence of extinction coefficient (at l ¼ 550 nm) for the Li+ doped Gd2O3:Eu3+ thin films annealed at different temperature and the inset shows the corresponding transmission spectra. The transmittance measurements from the inset of Fig. 6, it is evident that the films are highly transparent in the visible region and the transmittance values ranges from  64 to  78% except for the film annealed at 973 K. The absorption edge of the films annealed at 873 and 973 K are found to be slightly shifted towards the higher wavelength region (red shift), which is an indication of decrease in optical band gap of the films. Fig. 7 shows the wavelength dependence of extinction coefficient for the Li+ doped and undoped Gd2O3:Eu3+ thin films annealed at 973 K and the undoped Gd2O3:Eu3+ thin films annealed at 1173 K. The inset shows the corresponding transmission spectra along with that of pure Gd2O3 film annealed at 1173 K. The undoped Gd2O3:Eu3+ thin films show a higher transmittance (  86%) as compared with Li+ doped Gd2O3:Eu3+ thin films. The reduction in transmittance with Li+ doping may be due to the

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Fig. 5. Atomic force microscopic images of Li+ doped Gd2O3:Eu3+ thin film samples annealed at (a) 773 K, (b) 873 K, (c) 973 K and (d) 1073 K.

increased surface roughness and oxygen deficiency of the films and is also evident from AFM analysis. The surface roughness and stoichiometry play a crucial role in determining the luminescence brightness of a thin film phosphor [21]. Hence the extinction coefficient derived from the transmission spectra of the films is of technologically important. The extinction coefficient k was calculated using the relation k¼

al 4p

(3)

The extinction coefficient for the Li+ doped and undoped Gd2O3:Eu3+ thin films annealed at different temperatures are listed in Table 1. There can be two competitive mechanisms responsible for the observed variation of extinction coefficient with annealing temperature and Li+ incorporation namely (a) surface roughness and (b) stoichiometry of the films [53]. In Li+ doped Gd2O3:Eu3+ thin films the extinction coefficient decreases from 0.1439 to 0.1267 with increase in annealing temperature in the range from 300 to 873 K. This is an indication of considerable improvement in stoichiometry of the films with in

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Fig. 6. Wavelength dependence of extinction coefficient in Li+ doped Gd2O3:Eu3+ thin films: (a) as deposited; annealed at (b) 773 K, (c) 873 K, (d) 973 K and (e) 1073 K.

Fig. 8. Room-temperature PL spectra from Li+ doped Gd2O3:Eu3+ thin films: (a) as deposited; annealed at (b) 773 K, (c) 873 K, (d) 973 K and (e) 1073 K. The inset shows the corresponding excitation spectra.

3.5. Luminescent properties

Fig. 7. Wavelength dependence of extinction coefficient in (a) undoped Gd2O3:Eu3+ and (b) Li+ doped thin films annealed at 973 K and (c) undoped Gd2O3:Eu3+ thin films annealed at 1173 K. The inset shows the corresponding transmission spectra with (d) pure Gd2O3 film annealed at 1173 K.

the temperature range. But at 973 and 1073 K the extinction coefficient increased about 39.7 and 50.51% respectively from 0.1267 at 873 K. The increased extinction coefficient at 973 K with high stoichiometry may be due to the increase of optical scattering and optical loss that results from the increased surface roughness [54]. At 1073 K the surface roughness is small as compared with, the film annealed at 973 K and as is obtained from AFM analysis in Section 3.3, but the increased extinction coefficient reveals that the film can become less stoichiometric and can contain a large number of oxygen vacancies created as a result of the loss of volatile lithium components from the film. In undoped Gd2O3:Eu3+ thin film, annealed at 973 K shows a very low value of extinction coefficient and is a qualitative indication of excellent surface smoothness. Li+ doped Gd2O3:Eu3+ thin film, annealed at 973 K shows a higher value of extinction coefficient as compared with undoped Gd2O3:Eu3+ thin film, annealed at 1173 K. The higher value of extinction coefficient may be due to the increased surface roughness and reduced stoichiometry as a result of Li+ doping. These results fairly substantiate the XRD and AFM implications.

Fig. 8 shows the room-temperature PL spectra from Li+ doped Gd2O3:Eu3+ thin films annealed at different temperatures and the inset shows the corresponding excitation spectra. The excitation spectra were obtained by monitoring the emission of Eu3+ due to the transition 5 D0 –7 F2 at 612 nm. It can be seen from the inset of Fig. 8, that the excitation spectrum consists of a broad intense band with a maximum at 265 nm and a shoulder around 274 nm whose intensity increases with the increase in annealing temperature up to 973 K and then decreases. The increased excitation intensity may be due to the better crystallinity of the sample at higher annealing temperatures up to 973 K. The sample annealed at 1073 K, the excitation intensity become very low and it may be due to the presence of monoclinic phase of Gd2O3 host lattice and in monoclinic system the energy transfer between host Gd2O3 to the doped Eu3+ ion is not so efficient as compared with cubic Gd2O3. The general f–f transition lines of Eu3+ and Gd3+ in the longer wavelength region have not been observed due to their relatively weak intensity compared to the strong Gd2O3 host excitation band. Considering the transmission spectra for pure Gd2O3, Eu3+ doped Gd2O3 and Li+doped Gd2O3:Eu3+ films shown in the inset of Fig. 7, the 265 nm (similar to the broad intense band with a maximum at 265 nm present in the excitation spectrum) can be attributed to the charge transfer band from O2 to Eu3+ ions and the peak at 274 nm to the f–f transition of Gd3+ ions. Fig. 9 shows the roomtemperature PL spectra of undoped and Li+ doped Gd2O3:Eu3+ thin films annealed at different temperatures. The inset shows the corresponding excitation spectrum. A close examination of the excitation spectrum reveals that Li+ doped Gd2O3:Eu3+ thin films annealed at 973 K show enhanced excitation intensity as compared to the undoped Gd2O3:Eu3+ thin films annealed at 973 and 1173 K. On comparing Li+ doped and undoped Gd2O3:Eu3+ thin films annealed at 973 K, the enhanced excitation intensity may be due to the better crystallization as well as the presence of a number of oxygen vacancies created due to Gd3+ sites occupied by the smaller Li+ ions, which might act as a sensitizer for the energy transfer to the rare-earth ion owing to the strong mixing of charge transfer states [55]. The undoped Gd2O3:Eu3+ thin films annealed at 1173 K show better crystallinity, as compared with Li+ doped Gd2O3:Eu3+ thin films annealed at 973 K, but the Li+ doped film

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Fig. 10. The variation of PL intensity and crystalline size in Li+ doped Gd2O3:Eu3+ thin films as a function of annealing temperature.

Fig. 9. Room-temperature PL spectra of (a) undoped and (c) Li+ doped Gd2O3:Eu3+ thin films annealed at 973 K and (b) undoped Gd2O3:Eu3+ thin films annealed at 1173 K. The inset shows the corresponding excitation spectrum.

shows enhanced excitation intensity which may result only from the presence of a number of oxygen vacancies. Excitation into the Gd2O3 host band at 265 nm yields the emissions corresponding to f–f transitions of Eu3+ ion which were dominated by the hypersensitive 5 D0 –7 F2 transition at 612 nm. The characteristic PL emission involves transitions from the excited 5 D0 level to the crystal field split 7 FJ manifolds of the 4 F6 electronic configuration. In europium, the 5 D0 –7 FJ emission is a very sensitive probe of the crystal field around the Eu3+ site. In Eu3+, 5 D0 –7 F1 emission is an allowed magnetic dipole transition and 5 D0 –7 F2;3;4 is a forbidden electric dipole transition (parity selection rule) [56]. This selection rule can be relaxed when Eu3+ is placed in a host lattice like Gd2O3, which lacks in inversion symmetry [17]. It is well known that in a cubic Gd2O3 lattice two distinct sites are available for rare-earth doping, i.e., sites with C2 or C3i (or S6) point group symmetry [57,58]. The rare-earth ion occupying C3i site possess a center of inversion symmetry making the 5 D0 –7 F2;4 optical transition strictly forbidden. Therefore the dominant 5 D0 –7 F2;4 rare-earth emission lines originate from forced electric dipole transitions of the Eu3+ ion occupying C2 sites with a lack of inversion symmetry and from allowed electric dipole transitions. More specifically the forced electric dipole transitions for Eu3+ (5 D0 –7 F2;4 ) are hyper sensitive to the host crystallographic symmetry [56]. The monoclinic Gd2O3 provides three different Cs crystallographic sites for the Eu3+ ion [18]. These three sites give rise to a majority of the 5 D0 and 7 F2 stark levels which produce numerous peaks in the range 600–640 nm even though our measurement was not sufficient to resolve them. Hence Eu3+ ions in different Gd2O3 crystal structures show different emission lines. From Fig. 8 it is seen that in Li+ doped Gd2O3:Eu3+ thin films, the photoluminescence spectra of the films were found to increase in intensity with annealing temperature up to 973 K. This may be due to the combined effects of good crystalline structure which increases the oscillator strength for optical transitions, the improvement in crystalline size results in less light scattering at the surfaces and interfaces which in turn improves the probability of radiative transitions of the excited activated ions and finally a larger amount of Eu3+ ions were embedded into the matrix with elevated annealing temperatures. The variation of PL intensity as a function of crystalline size (dc ), particle size (d p ) and surface roughness also with annealing temperature are shown in

Fig. 11. The variation of PL intensity and particle size in Li+ doped Gd2O3:Eu3+ thin films as a function of annealing temperature.

Figs. 10–12 respectively. A closer analysis of these figures with Xray diffraction data reveal that, higher the crystalline/particle size, thicker the film, rougher the surface and less monoclinic phase leading to a strong PL emission at 612 nm. In Fig. 8, Li+ doped Gd2O3:Eu3+ thin films annealed at 1073 K show lower lumines-

Fig. 12. The variation of PL intensity and RMS roughness in Li+ doped Gd2O3:Eu3+ thin films as a function of annealing temperature.

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cence due to the presence of monoclinic phase. The high intensity of 624 nm peaks as compared to 612 nm peaks may be attributed to the monoclinic phase of Gd2O3 matrix, and the identical crystallographic structure identified by the XRD and MicroRaman analysis in Sections 3.1 and 3.2. It agrees well with the result that the monoclinic system shows a considerably lower luminescence than the cubic system [17]. From Fig. 9 the brightness of Li+ doped Gd2O3:Eu3+ films annealed at 973 K were increased by a factor of 3.04 in comparison with that of Gd2O3:Eu3+ films annealed at same temperature and 1.18 times increased in comparison with undoped Gd2O3:Eu3+ films annealed at 1173 K. In comparison with Li+ doped and undoped films annealed at 973 K, the improvement in PL brightness with the Li+ doping may be attributed to the following reasons. The crystallinity become improved and the crystalline size become twice, the improved crystallinity leading to higher oscillating strength for optical transitions [59] and the increased crystalline size reduces the grain boundary density which may act as sources of dissipation, adsorbing and/or scattering light generated inside the film that resulted in lower PL brightness. The increased surface roughness with Li+ doping may reduce the loss of emitted light due to internal reflections within the film. By Li+ doping these factors come to play at lower temperature of about 973 K. The additional factors which contribute the enhanced luminous intensity only by Li+ doping are the formation of a number of oxygen vacancies which might act as a sensitizer for energy transfer to the rare-earth ion owing to the strong mixing of CT states and the flux effect of Li+ ions as discussed in Section 3.1. Undoped Gd2O3:Eu3+ films annealed at 1173 K show better crystallinity, and grain size as compared with Li+ doped Gd2O3:Eu3+ films annealed at 973 K, but the enhanced PL intensity may only due to the above mentioned additional factors. Li+ doped Gd2O3:Eu3+ films annealed at 973 K, the FWHM of the prominent peak at 612 nm becomes higher as compared with other doped and undoped films at different annealing temperatures. In the surface, Eu3+ ion should posses a different disordered environment compared to those in the center of a bulk crystalline material leading to inhomogeneous broadening of the emission at elevated temperatures [25] and with Li+ doping. The transition of the Eu3+ ion is mainly affected by the symmetry of the crystal field. The 5 D0 –7 F2 transition is electricdipole allowed and its intensity is sensitive to the local structure surrounding the Eu3+ ions. On the other hand, the 5 D0 –7 F1 transition is magnetic-dipole allowed and its intensity shows very little variation with the crystal field strength acting on the Eu3+ ion. Therefore, the intensity ratio of the electric-dipole to magneticdipole transition is widely used for the study of the chemical bond of anions coordinating the rare-earth ions [60]. Fig. 13 shows the resolved emission spectra of the crystal-field-splitting lines of 5 D0 – 7 F0 and 5 D0 –7 F1 transitions at various annealing temperatures and with Li+ doping. The peak around 580 nm can be attributed to the 5 D0 –7 F0 transition, where as the 5 D0 –7 F1 transition should have three Stark splitting peaks. As shown in Fig. 13, the lines of 5 D0 –7 F1 transition can be resolved into three Guassian components e , e0 and eþ [60]. From a detailed analysis of the Stark splitting peaks originated from the 5 D0 –7 F1 transition, it is obvious that the Guassian components are very much sensitive to the crystallinity and phase of the matrix, processing temperature and Li+ incorporation in to the matrix. From Fig. 13, the e0 component is only present in the as deposited and Li+ doped Gd2O3:Eu3+ film annealed at 1073 K. This may be due to the poor crystalline quality of the former and the predominance of monoclinic structure of the latter. In the case of Eu3+ ions with S6 symmetry, the 5 D0 –7 F1 magnetic dipole transition is dominant. The degree of site asymmetry is defined as the intensity ratio of the 5 D0 –7 F2 transition to the 5 D0 –7 F1 transition, the local symmetry of

surrounding impurity ions, i.e., the occupancy ratio of the two different sites (ORS) could be obtained [18,61]. Fig. 14 represents the variation of ORS and PL intensity of Li+ doped and undoped Gd2O3:Eu3+ thin film samples as a function of annealing temperature. From the figure, in Li+ doped samples it seems that the Eu3+ and Li+ ions are not assumed to occupy C2 and S6 sites in a statistical way in accordance with the variation in annealing temperature and a similar observation is made by Shin et al. during the study on the effect of Li+ incorporation concentration in Gd2O3:Eu3+ phosphors [62]. As the annealing temperature goes higher and higher, more and more Eu3+ and Li+ ions are introduced into the S6 site, which will enhance the 5 D0 –7 F1 magnetic dipole transition and hence it reduces the values of ORS at high temperature region in Li+ doped films. Oomen et al. treated occupancy ratio (ORS) as asymmetry ratio R [63], and were obtained as given in Table 2. Some authors suggested that higher R (ORS) value originates from a more distorted local environment of Eu3+ ions and the intensity of 5 D0 –7 F2 transition strongly depends on the asymmetry of Eu3+ site, i.e., the higher value of asymmetry ratio gives high PL intensity due to 5 D0 –7 F2 transition [64,65]. Also higher value of R confirms that a larger portion of the Eu3+ ions resides near or at the surface of the nanocrystals due to their higher surface/volume ratio. From Fig. 14 it is found that the value of asymmetry ratio varies in a similar manner with PL intensity in pure Gd2O3:Eu3+ thin films. But in Li+ doped Gd2O3:Eu3+ films the value of asymmetric ratio decreases with increase in annealing temperature. On the other hand the PL intensity was found to be enhanced up to 3.04 times as compared with Gd2O3:Eu3+ thin films annealed at 973 K. The asymmetric ratio is being widely used for probing the distortion of local environment around Eu3+ ions in the lattice and the decrease in the values of asymmetry ratio at elevated temperatures in Gd2O3:Eu3+ thin films with Li+ doping indicates a reduction in distortion in the local environment around Eu3+ ions. On the basis of Judd-Ofelt theory a higher asymmetry ratio corresponds to a larger intensity parameter V2 , implying a stronger covalency of Eu–O bonds [60]. The asymmetry ratios were 3.49 and 1.9 for undoped and Li+ doped Gd2O3:Eu3+ thin films respectively. These results indicate that the covalency of Eu–O bond in Li+ doped Gd2O3:Eu3+ thin films could be weakened by annealing at elevated temperatures and with Li+ doping in Gd2O3:Eu3+ matrix. In undoped Gd2O3:Eu3+ thin films, the absence of Li+ could be attributed to the oriented growth of the film, shortening the Eu–O bond length and improving the covalency of Eu–O bond. In this work the emission spectrum was found to be highly intense for the Li doped films. Irrespective of doping and deposition conditions the emission peak was found to be at 612 nm when excited at 265 nm. The intensity of Eu3+ in RE2O2 nanocrystals with an average size larger than 10–20 nm should not differ significantly from that of the bulk samples [66] highlights the role of Li+ in the present study. A significant shift in the excitation spectrum towards the longer wavelength region was observed in all the samples with reference to the values found in some other reports [67,68] and it may due to an increase in Eu–O bond length in these samples [69]. Jun Yang et al. reported emission spectra occurring from the 5 D0 –7 F2 transitions at 610 nm when excited at 257 nm for Gd2O3:Eu3+ products derived through a synthetic route [68]. Liu et al. reported emission at 611 nm for Gd2O3:Eu3+ nanotubes prepared by hydrothermal synthesis and they have attributed luminescent properties to the morphology and the lack of low energy phonon modes in the nanotubes [67]. It should be noted that our room temperature line positions were slightly different from those by Liu et al. [67] (where the spectra were collected at 10 K) and those in Yang et al. [68], a small position difference from the emission lines can be attributed to the variations in the nature of the samples and measurement conditions.

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Fig. 13. Resolved emission spectra of the three crystal-field-splitting lines of the 5 D0 –7 F0 and 5 D0 –7 F1 transition; Li+ doped Gd2O3: Eu3+ films (a) as deposited; annealed at (b) 773 K, (c) 873 K, (d) 973 K, (e) 1073 K; Gd2O3:Eu3+ films annealed at (f) 973 K and (g) 1173 K.

CIE 1931 XY chromaticity diagram in Fig. 15 was also plotted for further analysis of the thin film phosphors. The CIE 1931 graph comprises triangle of R (red), G (green), and B (blue). The purity of red increases with increasing  coordinates. The position of a colour in the diagram is called the chromaticity point of the colour [70]. The CIE coordinates were calculated by integrating emission counts from 580 to 750 nm wavelength range. The chromaticity coordinates and asymmetric ratio (R) are given in Table 2. According to the CIE plot the average value of the Li+ doped and

undoped Gd2O3:Eu3+ thin film coordinates falls with in the orangered region. The more pronounced deviation from the red colour coordinates of the standards of the film based on Gd2O3 host should be mostly attributed to the following reasons. (i) Lower crystallinity of the samples due to lower processing temperatures, (ii) the presence of monoclinic phase in Li+ doped Gd2O3:Eu3+ thin film samples annealed at 1073 K and (iii) the reduction in the value of asymmetry ratio with increase in annealing temperature in Li+ doped Gd2O3:Eu3+ thin film samples and with Li+ doping.

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4. Conclusion

Fig. 14. The variation of asymmetry ratio (R) or ORS and PL intensity as a function of annealing temperatures.

Table 2 Variation of asymmetry ratio and chromaticity co-ordinates as a function of annealing temperature and Li+ doping. Sample code

AT (K)

Li doped Gd2O3:Eu3+films 1 300 2 773 3 873 4 973 5 1073 Gd2O3:Eu3+ films 6 7

973 1173

R

CIE 1931 co-ordinates x

y

1.6565 3.2330 2.1900 1.9000 1.1985

0.530 0.574 0.582 0.577 0.536

0.464 0.425 0.418 0.422 0.463

2.8900 3.4900

0.562 0.604

0.437 0.360

Nanostructured Gd2O3:Eu3+ and Li+ doped Gd2O3:Eu3+ thin film phosphors have been deposited on quartz substrate using pulsed laser deposition technique. The X-ray diffraction and Micro-Raman analysis suggest that the crystalline phase and crystalline size of the film are very crucial factors to determine the PL brightness. SEM analysis reveals that with increase in annealing temperature Gd2O3:Eu3+ films were found to transform into crystalline form with different degree of ordering. AFM images of the Li+ doped Gd2O3:Eu3+ films annealed at different temperature show a spontaneous ordering of the nanocrystals distributed uniformly all over the surface, with a hillocks like self-assembly of nanograins especially in the film annealed at 973 K, which exhibits enhanced photoemission about 3.04 times as compared with undoped Gd2O3:Eu3+ thin film phosphors enabling it to be use in field emission display applications with high resolution and tunable chromaticity. The present investigation gives a more light into the dependence of photoluminescence with microstructural properties by relating the variation of extinction coefficient of films grown under different deposition conditions with it. A detailed analysis of the Stark splitting peaks originated from the 5 D0 –7 F1 transition, it is obvious that the Guassian components are very much sensitive to the crystallinity and phase of the matrix, processing temperature and Li+ incorporation in to the matrix. From the CIE diagram by varying the processing conditions and the incorporation of Li+ into Gd2O3:Eu3+ matrix, it is possible to tune chromaticity coordinates starting from yellow to red region for the use of saturated colours in full colour display applications. Acknowledgments The authors are grateful to Professor Ajay Gupta, Dr. Sreepathy, Dr. Ganesan, Dr. Vasant Sathe, Dr. Raghavendra Reddy and Dr. Phase of the UGC-DAE Consotium, Indore Centre, for their valuable suggestions and help in the measurements. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21] [22]

+

Fig. 15. The CIE 1931 chromaticity diagram for Li doped and undoped Gd2O3:Eu thin films annealed at different temperatures.

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